This invention relates generally to vehicle rearview mirror systems and more particularly to mirror systems utilizing electrochromic reflective elements. The invention is especially adapted to externally located rearview mirrors utilizing an all solid-state electrochromic reflective element, having a continuously variable reflectance surface.
Use of auxiliary heaters to serve as defrosting means for exterior vehicle rearview mirrors is well known in the art. However, their purpose is merely to melt ice or evaporate moisture on the mirror surface, and particularly that built up while the vehicle was parked. Thus, prior art heaters for mirrors are intended to heat the mirror to at least 0.degree. C. in as rapid a time as possible. Because the de-icing process is aided by already melted run-off water that tends to assist de-icing of peripheral areas, and because the center region of the mirror is the most utilized area, auxiliary heating means intended for defrosting purposes are more concerned with a rapid heating of the exposed front surface of the mirror, particularly its central region, rather than a uniform distribution of heat across the reflective element.
Although heating pads and resistance films can be designed to provide uniform power distribution suitable for de-icing purposes over the front surface of the mirror, commercially available products built according to the teachings of the prior art do not heat the reflective element in a truly uniform manner. However, because of the necessity for producing large quantities of heat as opposed to an even distribution, such prior art devices have proven satisfactory for the de-icing function.
Electronically dimming vehicle rearview mirror systems tend to utilize either liquid crystal or electrochromic reflective elements. Electrochromic reflective elements are further divided into all solid-state elements, such as disclosed in U.S. Pat. No. 4,712,879 issued to Niall R. Lynam for ELECTROCHROMIC MIRROR, and electrochemichromic elements such as disclosed in co-pending commonly assigned U.S. Pat. No. 5,140,455 for HIGH PERFORMANCE ELECTROCHEMICHROMIC SOLUTIONS AND DEVICES THEREFORE issued to Desaraju V. Varaprasad, Niall R. Lynam, Hamid R. Habibi and Padma Desaraju. The liquid crystal elements and the electrochemichromic elements are fluid-like at room temperature but greatly increase in viscosity and can even solidify at very low temperatures with a consequent decrease in performance. Accordingly, heating means are often provided for such elements when used in vehicle rearview mirror systems, such as disclosed in U.S. Pat. No. 4,584,461 for a PLANAR HEATER FOR LIQUID CRYSTAL ELEMENT, issued to Toru Teshima et al. The all solid-state electrochromic element stands in contrast to the liquid crystal and electrochemichromic elements in that it utilizes multiple layers of inorganic metal oxide thin films deposited on a transparent substrate rather than a solvent-based liquid solution. Thus all layers are solid at all temperatures. However, cold climate operation is still a problem in that the electrochromic performance (the depth of dimming achieved, the speed of coloration and/or bleach, etc.) can be detrimentally effected at low temperatures. In this regard, all solid-state electrochromic mirrors have a particular disadvantage in that ion and electron conduction and transport through the thin films, important to the electrochromic coloration and/or bleaching, is particularly slowed at low temperatures (below ambient in general and 5.degree. C. or thereabouts in particular). The net effect is that, at low temperatures, these solid-state electrochromic devices are significantly slowed and, if temperature gradients exist across the mirror surface, uneven coloration and/or bleaching can be seen. Such effects are consumer undesirable and detract from the glare protection performance of the units.